Due to their high strength and toughness, attention is now paid as the structural materials to partially stabilized zirconia sintered bodies (hereinafter referred to as the PSZ sintered bodies) in which the tetragonal zirconia of a metastable phase remains, as well-known in the art, or composite sintered bodies of the partially stabilized zirconia with other ceramics. Similar attention is also paid to the fact that they can be used as the function elements that are used under severe conditions due to the oxygen sensitivity zirconia per se possesses. However, the PSZ sintered bodies have offered a problem in connection with durability, and have been only used in limited fields. This is because the gradual transformation of the tetragonal zironia to the monoclinic zirconia takes place at a temperature of 200.degree.-400.degree. C. with the accompanying drop of mechanical strength. In order to provide a solution to this problem, it has been proposed to apply a water-resistant coating film on the surface of the PSZ sintered bodies with effectiveness. Such proposal has been disclosed in an article entitled "Low Temperature Deterioration of PSZ", Preliminary Transactions of the Spring Conference of 1984, page 122, Japan Society of Powder and Powder Metallurgy. The means disclosed in the aforesaid Preliminary Transactions has been based on the findings that the transformation of the tetragonal to monoclinic zirconia results from moisture present in an atmosphere.